Here are free options to optimize PDF documents on various operating systems:

On Mac OS X

On the Mac, it is possible to compress PDF files by following the steps below:

Open the PDF in Preview: Control-click on the PDF file, choose “Open With” and select “Preview” in the menu that appears on the screen.
Choose “Save As” in the “File” menu. Enter a name for the reduced PDF and specify a save location in the “Save As” window.
Click on the pull-down menu next to “Quartz Filter.” Select “Reduce File Size.” Click “Save” to complete the process.